1. Field of the Invention
The present invention relates to a blood separating composition for use in the centrifugal separation of the blood components such as serum and plasma, wherein the difference in the specific gravity between the blood components are utilized.
2. Prior Art
There are known various types of the blood separating compositions which are used to separate the blood components from one another. The main ingredient in the compositions is a gel material, for example a silicone oil, a chlorinated polybutene or an acrylic polymer. A thixotropic agent such as silica or clay is usually added to the main ingredient of the blood separating composition, which is to be previously set in place on the bottom of a blood collection tube so as to provide a blood testing receptacle. A blood sample will be taken in the blood testing receptacle, and be maintained in this state for a given period of hours before being centrifuged. The centrifugal force causes the blood separating composition to be fluidized, and it will gradually ascend off the bottom because its specific gravity is intermediate the specific gravity of the serum or plasma and that of the blood cells. Thus, a barrier is formed between the layer of serum or plasma and the layer of blood cells, to thereby separate the former layer from the latter.
The prior art blood separating composition containing the thixotropic agent has been problematic about its thixotropic property, as will be described below.
As well known in the art, thixotropy is such a phenomenon that a material which shows in its stationary state a higher viscosity will become less viscous and substantially flowable under the influence of an external force or stress applied thereto, provided that the force exceed a certain strength. Upon removal of the external force, this material however will recover gradually and slowly its high viscosity. In other words, a certain length of time is necessary for such a material to change from its less viscous state back into its highly viscous state, after the external force is removed.
The certain strength of the external force for changing the material from its stationary state to its dynamic one is called a yield stress. Likewise, another yield stress will be observed also when the material returns from its dynamic state to its stationary state.
The cause of this mechanism is considered to be a weak intermolecular force such as the hydrogen bonds which may take place between the molecules of the gel material as main ingredient, between the molecules of the added thixotropic agent, and/or between the former molecules and the latter ones.
Such a thixotropic property will play an important or decisive role in the blood separation by the blood separating composition, as will be described below.
The viscosity and the yield stress when the blood separating composition changes from its stationary state to its dynamic state gives influence on its ascendability and/or flowability. In detail, the composition resting on the bottom of a vacuum blood collection tube will move towards an upper end thereof closed with a rubber plug, in a manner depending on the viscosity and yield stress. Further, the blood separating composition will ascend to the upper end at a timing also depending on the viscosity of said composition in its dynamic state. On the other hand, the viscosity and another yield stress of the blood separating composition returning from its dynamic state to its stationary state will affect the self-standing property or strength, and/or the reversibility of the barrier after the centrifugal separation process.
In more detail, the following problems will occur when the centrifugal separation is carried out after a proper period of time for a blood collection tube which contains on its bottom the blood separating composition and a given volume of a blood to be tested.
If the blood separating composition will not become flowable to ascend off the bottom of the blood collection tube in spite of the centrifugal force applied, then any barrier will not be formed between the layer of serum or plasma and the layer of blood cells, thus causing a direct contact of the former with the latter. If the barrier which is formed by the flowable and ascending blood separating composition is not self-standing but peels off the wall of said blood collection tube, then there will occur not only the direct contact of the two layers, but also some blood cells will remain above the barrier in the phase of the serum or plasma. If the blood separating composition on the bottom of the vacuum blood collection tube will incidentally move towards the upper end thereof closed with a rubber plug, then a significant amount of the composition will remain the phase of serum or plasma, or will stick to the rubber plug, thereby causing the contamination of said phase or the difficulty in obtaining a pure sample of serum or plasma.
Almost all of the prior art blood separating compositions are prepared taking no account of such a thixotropy, and consequently, none of them is free from the abovementioned problems.